spec: clarify meaning of embedded unexported fields/methods

[griesemer]

See issues 1536 and 1550 for programs whose legality is not obviously clear from the
spec.

[rsc]

Comment 1:

Labels changed: added priority-later.

[rsc]

Comment 2:

Labels changed: added priority-go1.

[robpike]

Comment 3:

Owner changed to builder@golang.org.

[rsc]

Comment 5:

Labels changed: added go1-must.

[griesemer]

Comment 6:

Owner changed to @griesemer.

Status changed to Started.

[griesemer]

Comment 7:

http://golang.org/cl/5711043

[griesemer]

Comment 8:

This issue was closed by revision 103c9db.

Status changed to Fixed.